I suspect that you have some contaminant in the exhaust which is a mixture of condensation build up and something else, hence the smell.
Give the guys that did your conversion a call and ask them what they think about it. If they say that they have never had such issues etc. then try the following:
Get hold of some Forte Fuel additive which is great for clearing the system of such things, put a third of it in with around £25 fuel, switch the car to Petrol and take it on a 50 miles spin on the motorway. That should clear it up.
Start the car the next morning on petrol and see how it is. If it seems to be better then what you will need to do is start the car the following morning on gas and see what's what.
Is your Evo de-catted???
El